The present invention relates to a method of making electrically resistive sheets of material; and more particularly, to an improved method of making sheet material having electrically conductive fibers disposed randomly at a selected density for producing a resistive network that has uniform dielectric properties without polarization dependence.
Resistive sheet material is useful in removing precipitated static electricity from aircraft surfaces, such as radomes, for example; and also, in reflecting selected bands of frequencies, while passing other frequencies. The particular characterisics of the sheet material determine the frequency bands that are selected.
An insulative sheet material having small conductive fibers, such as carbon or metal, bonded on one surface thereof, is sometimes used as a resistive sheet. The type of fibers, the length and diameter of the individual fibers, the density of the fibers, and the uniformity of distribution on the surface of the material, all affect the resistivity and characteristics of the material. In order to be effective, however, the sheet material should have uniform resistivity in all directions; and also be able to be manufactured so that the desired characteristics can be repeatedly obtained from one batch of material to another.
Small electrically conductive fibers, such as carbon or metal fibers, when dispersed in a manner where they are free to individually orient themselves, tend to attract each other and align themselves in a particular direction. This alignment produces a resistive sheet material that exhibits polarization dependent electrical characteristics which adversely affects the ability of certain frequencies, particularly microwave, to pass through the material. Further, by their very nature, the uniformity of distribution, and the accuracy of selected densities of small fibers or filaments (1/4" to 3/4" in length and eight to twelve microns in diameter) are difficult to control.
Heretofore, in order to minimize the problem of polarization and to control the density of the fibers, a sheet of resistive material was made from a plurality of individual laminated layers the number of which depended on the desired density; and with each sheet oriented relative to the other in order to neutralize polarization dependencies. Also, a known method for overcoming the aforementioned problems involved spraying a solution of an anti-static surface coating containing the fibers directly on the surface of the sheet material. Although, the spraying of the coating/fiber mixture offset the tendency of the fibers to align themselves, it was difficult to control accurately, the density and uniformity of distribution of the fibers, as well as to insure that the fibers were properly bonded to the surface.